Beneficiated phosphate rock from Florida typically contains 3 to 4% fluorine values. Phosphoric acid can be produced from this rock by treating it with sulfuric acid. Part of the fluorine present in the rock is evolved as silicon tetrafluoride and gaseous hydrofluoric acid, which upon scrubbing with pond water, form a dilute fluosilicic acid. By efficient design of scrubbers, it is possible to continuously produce a solution containing about 20% fluosilicic acid. The acid is usually contaminated with impurities, including 1,000 to 4,000 parts per million of P.sub.2 O.sub.5.
It is desirable to recover the fluorine values present in the fluosilicic acid as anhydrous hydrofluoric acid. This is because hydrofluoric acid can be an important source of revenue. In addition, the presence of fluorine in the pond water presents an environmental pollution problem.
Many processes have been developed for concentration of dilute fluosilicic acid solutions, and preparation of hydrofluoric acid from the concentrated fluosilicic acid. Such attempts are described in U.S. Pat. Nos. 3,645,678; 3,645,679; 3,689,216; 3,855,399; 3,278,265; 3,218,124; 3,256,061; 3,140,152; 3,914,398; 3,537,817; 3,758,674; German Offen. Nos. 2,035,300, 2,032,855, and 2,248,149; and French Pat. No. 7,034,470. However, these processes suffer from one or more disadvantages. Disadvantages of these processes include operation at excessively high temperatures or under severe conditions, use of an excessive number of processing steps, consumption of uneconomical quantities of raw materials, production of undesirable byproducts, production of contaminated hydrogen fluoride, low yield of hydrogen fluoride, and considerable expenditure, both in terms of operating expense and initial capital investment. For example, U.S. Pat. Nos. 3,218,124 and 3,689,216 describe a process where fluosilicic acid solutions are treated with concentrated sulfuric acid to liberate silicon tetrafluoride and hydrogen fluoride, which are then separated. The silicon tetrafluoride is hydrolyzed to fluosilicic acid which is recycled and SiO.sub.2 which is removed. Two disadvantages of this process are that a large volume of concentrated sulfuric acid is required per unit of fluosilicic acid and the splitting of the fluosilicic acid must be carried out at relatively high temperatures. This can result in severe corrosion of equipment.
U.S. Pat. No. 3,256,061 describes a process whereby fluosilicic acid is neutralized with ammonia, producing ammonium fluoride and silica. The silica is separated by filtration, and the ammonium fluoride is concentrated to a molten state constituting NH.sub.4 F-NH.sub.4 HF.sub.2, which when treated with concentrated sulfuric acid produces hydrogen fluoride. Ammonia remains in the sulfuric acid and is sent to a phosphate acidulation unit. The chief drawbacks of this process are the requirement to recycle ammonia and the failure to remove any P.sub.2 O.sub.5 impurity in the fluosilicic acid. A similar process is described in U.S. Pat. Nos. 3,914,398 and 3,537,817.
Therefore, there is a need for a simple, high yield process for recovering high purity hydrogen fluoride from phosphoric acid plant process streams.